Ask him what they were doing.
If he says they were "studying for a test" ask him for what class. If you don't have that class, ask him WHO exactly it was.

Go up to her.
If she tries to run; they did it.
If she smiles when she sees you; they did it.
If she acts as if you're not there; they did it.
If she says "i don't know what you're talking about"; they did it.
if she tells you;they didn't....or it just might be a cover story, so;they still could have done it.